There are times when various words can both be used to describe one particular kind of animalsanimal. The animal one word specifies is a bit different from the other(s), but in most cases there's plenty of overlap and most people struggle to tellfew know the little things about them apartthat differentiates them.
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crocodile and alligator
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turtle and tortoise
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shrimp, /prawn, and lobster
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crab, lobster, and crayfish
EDIT: I know, the examples are a bit confusing. I wrote this in a hurry, thinking that I'll edit it later on (which I have done). The species/catagory these words fall into is the same.